To understand how your breast cancer is likely to behave and which treatments are most likely to stop it, your doctors will determine several traits of your cancer at the time of your diagnosis or soon after. These include:
- The type of breast cancer you have
- The grade of your breast cancer
- Whether your cancer is hormone-receptor positive
- Whether your cancer is HER2 positive
Breast cancers fall into these two main categories:
- In situ breast cancers, also called noninvasive breast cancers: These are cancers confined to breast ducts (milk tubes) and lobules (milk-producing glands).
- Invasive breast cancers, also called infiltrating breast cancers: These cancers can start in the ducts or lobules, but then they spread into surrounding breast tissue. Invasive cancers may eventually become metastatic, meaning they find their way to other organs, like the bones, lungs, liver, and brain.
The American Cancer Society estimates that 231,840 new cases of invasive breast cancer were diagnosed among women and another 60,290 cases of in situ breast cancer were diagnosed in 2015.
In Situ Breast Cancers
Ductal carcinoma in situ (DCIS) is the most common type of noninvasive breast cancer. It is a condition in which abnormal cells are confined to the milk ducts in the breast. DCIS is considered early-stage cancer. Many women diagnosed at this early stage can be cured by removing the tissue that contains the tumor. Left untreated, DCIS can progress to invasive cancer. DCIS typically has no physical signs or symptoms. It is usually detected by a screening mammogram.
Invasive Breast Cancer
The following are the most common types of invasive breast cancer.
- Invasive ductal carcinoma (IDC): About 80 percent of invasive breast cancers are invasive ductal carcinomas, also called infiltrating ductal carcinomas. IDC begins in a duct in the breast and breaks through into the surrounding fatty tissue of the breast. From there, IDC can metastasize (spread) to other parts of the body through the bloodstream or lymphatic system.
- Invasive lobular carcinoma (ILC): Invasive lobular carcinoma is the second most common type of invasive breast cancer. ILC starts in the lobules and, like IDC, can spread to other parts of the body. It is also called infiltrating lobular carcinoma.
- Inflammatory breast cancer (IBC): Only about 1 to 3 percent of all women diagnosed with breast cancer have IBC. This rare cancer is invasive and may begin in either the ducts or the lobules. It is aggressive and progresses rapidly. In its early stages, IBC can be mistaken for infection because the symptoms are similar. However, most infections will respond to antibiotics, but inflammatory breast cancer will not.
Other rare types of breast cancer include Paget's disease of the nipple and phyllodes tumors.
The grade of an invasive breast cancer tumor refers to how closely the cancer resembles normal breast cells under a microscope. In general, a lower grade number indicates a slower-growing cancer that is less likely to spread, while a higher number indicates a faster-growing cancer that is more likely to spread.
Breast cancer grade is determined as part of a biopsy. There are three grades.
- Grade 1 (well differentiated): The cells in these cancers are relatively less abnormal looking under a microsope. The cells do not appear to be growing rapidly. They are arranged in small tubules.
- Grade 2 (moderately differentiated): The cells in these cancers are somewhat more abnormal.
- Grade 3 (poorly differentiated): The cells in these cancers look the most abnormal. They tend to grow and spread more aggressively.
The grade of a breast cancer is different from the stage. Stage is based on where the cancer has spread. Also read about breast cancer stages.
Your care team will try to determine other traits of your breast cancer when they check the sample of tissue removed during your biopsy or a sample taken when your entire tumor is removed surgically. Specifically, they will be looking for the following two traits:
- Hormone-receptor status: Most women with breast cancer have hormone-receptor positive (HR+) disease. This means their cancer cells have receptors where estrogen can attach (estrogen-receptor positive, or ER+, disease) or they have receptors where progesterone can attach (progesterone-receptor positive, or PR+, disease). Most women have both types of receptors. If your cancer is HR+, this helps doctors predict that your cancer will respond to hormonal therapy.
- HER2/neu status: Some women with breast cancer have HER2-positive disease. This means their cancer cells make too much of a protein called HER2/neu, which indicates that the cancer may be more aggressive. If your cancer is HER2 positive, this helps doctors predict that your cancer will respond to certain targeted biological therapies.
The term “triple-negative breast cancer” refers to breast cancers that are not estrogen-receptor positive, are not progesterone-receptor positive, and are not HER2-positive.